Optical fibers are key components in modern telecommunications. Optical fibers are thin strands of glass capable of transmitting an optical signal containing a large amount of information over long distances with very low loss. In essence, an optical fiber is a small diameter waveguide characterized by a core with a first index of refraction surrounded by a cladding having a second (lower) index of refraction. Light rays which impinge upon the core at an angle less than a critical acceptance angle undergo total internal reflection within the fiber core. These rays are guided along the axis of the fiber with minimum attenuation. Typical optical fibers are made of high purity silica with minor concentrations of dopants to control the index of refraction.
A typical optical fiber communications system comprises a source of optical input signals, a length of optical fiber coupled to the source and a receiver for optical signals coupled to the fiber. One or more amplifying devices are disposed along the fiber for amplifying the transmitted signal. Pump energy must be supplied to operate the amplifier. Contemplated optical fiber systems use digitally modulated optical signals at a wavelength of 1.55 micrometers and erbium-doped fiber amplifiers.
Such systems present a number of difficulties. One problem is the disposition of unused pump energy in a counter-pumped fiber amplifier (with two pump sources). If unused pump energy from one source is permitted to propagate down the fiber towards the other pump source, it can deteriorate the performance of the amplifier. Also, in any amplifier, amplified spontaneous emission generated by the interaction of the pump power with the rare-earth ions can act as noise and adversely affect system performance. In both these cases, it would be useful to have an in-fiber device that can effectively introduce a wavelength-dependent loss to increase the efficiency of the amplifier.
Another problem limiting the capacity of such systems is that the erbium-doped fiber amplifier has a characteristic spectral dependence providing different gain for different wavelengths. This spectral dependence poses a problem for contemplated multichannel wavelength division multiplexed (WDM) systems because different gains for different channels would lead to high bit error rates in some of the channels. In this case, a spectral shaping device would help flatten the gain spectrum of the amplifier.